With recent demands for small-sized refrigeration compressors with larger outputs for compressing Flon refrigerants that are used in air-conditioners, refrigerators, and the like, rotary compressors have been widely employed in replacement of conventional reciprocating compressors.
The rotary compressors have a high contact pressure and a high sliding speed in the sliding portion and also meet with a high discharge temperature. Therefore, lubricating oils to be applied are required to have excellent Flon stability, high wear resistance and assured extreme pressure resistance.
In order to improve wear resistance of lubricating oils, phosphoric esters, e.g., tricresyl phosphate and triphenyl phosphate, have hitherto been used as extreme pressure additives. However, results of evaluation of the lubricating oils containing these phosphoric esters through application to actual compressors revealed unfavorable phenomena that the wear of the sliding parts is rather promoted and that copper plating is promoted. It was confirmed that the wear is resulted from corrosion with hydrochloric acid which is a decomposition product of Flon and the decomposition of Flon is markedly accelerated in the presence of the phosphoric esters.
Further, Japanese Patent Application (OPI) No. 71464/76 (the term "OPI" herein used means "unexamined published application") discloses that refrigerating machine oils comprising long-chain alkylbenzenes as a base oil having incorporated therein dimethyl polysiloxane are excellent in heat stability. These oils, however, do not serve the purpose of improving wear resistance.